indirectly: they could be a form of “dark matter,” like that mentioned earlier, with sufficient gravitational attraction to
stop the expansion of the universe and cause it to collapse again.
About one hundred seconds after the big bang, the temperature would have fallen to one thousand million degrees,
the temperature inside the hottest stars. At this temperature protons and neutrons would no longer have sufficient
energy to escape the attraction of the strong nuclear force, and would have started to combine together to produce
the nuclei of atoms of deuterium (heavy hydrogen), which contain one proton and one neutron. The deuterium nuclei
would then have combined with more protons and neutrons to make helium nuclei, which contain two protons and
two neutrons, and also small amounts of a couple of heavier elements, lithium and beryllium. One can calculate that
in the hot big bang model about a quarter of the protons and neutrons would have been converted into helium nuclei,
along with a small amount of heavy hydrogen and other elements. The remaining neutrons would have decayed into
protons, which are the nuclei of ordinary hydrogen atoms.
This picture of a hot early stage of the universe was first put forward by the scientist George Gamow in a famous
paper written in 1948 with a student of his, Ralph Alpher. Gamow had quite a sense of humor – he persuaded the
nuclear scientist Hans Bethe to add his name to the paper to make the list of authors “Alpher, Bethe, Gamow,” like
the first three letters of the Greek alphabet, alpha, beta, gamma: particularly appropriate for a paper on the beginning
of the universe! In this paper they made the remarkable prediction that radiation (in the form of photons) from the
very hot early stages of the universe should still be around today, but with its temperature reduced to only a few
degrees above absolute zero (–273oC). It was this radiation that Penzias and Wilson found in 1965. At the time that
Alpher, Bethe, and Gamow wrote their paper, not much was known about the nuclear reactions of protons and
neutrons. Predictions made for the proportions of various elements in the early universe were therefore rather
inaccurate, but these calculations have been repeated in the light of better knowledge and now agree very well with
what we observe. It is, moreover, very difficult to explain in any other way why there should be so much helium in the
universe. We are therefore fairly confident that we have the right picture, at least back to about one second after the
big bang.
Within only a few hours of the big bang, the production of helium and other elements would have stopped. And after
that, for the next million years or so, the universe would have just continued expanding, without anything much
happening. Eventually, once the temperature had dropped to a few thousand degrees, and electrons and nuclei no
longer had enough energy to overcome the electromagnetic attraction between them, they would have started
combining to form atoms. The universe as a whole would have continued expanding and cooling, but in regions that
were slightly denser than average, the expansion would have been slowed down by the extra gravitational attraction.
This would eventually stop expansion in some regions and cause them to start to recollapse. As they were
collapsing, the gravitational pull of matter outside these regions might start them rotating slightly. As the collapsing
region got smaller, it would spin faster – just as skaters spinning on ice spin faster as they draw in their arms.
Eventually, when the region got small enough, it would be spinning fast enough to balance the attraction of gravity,
and in this way disklike rotating galaxies were born. Other regions, which did not happen to pick up a rotation, would
become oval-shaped objects called elliptical galaxies. In these, the region would stop collapsing because individual
parts of the galaxy would be orbiting stably round its center, but the galaxy would have no overall rotation.
As time went on, the hydrogen and helium gas in the galaxies would break up into smaller clouds that would collapse
under their own gravity. As these contracted, and the atoms within them collided with one another, the temperature
of the gas would increase, until eventually it became hot enough to start nuclear fusion reactions. These would
convert the hydrogen into more helium, and the heat given off would raise the pressure, and so stop the clouds from
contracting any further. They would remain stable in this state for a long time as stars like our sun, burning hydrogen
into helium and radiating the resulting energy as heat and light. More massive stars would need to be hotter to
balance their stronger gravitational attraction, making the nuclear fusion reactions proceed so much more rapidly that
they would use up their hydrogen in as little as a hundred million years. They would then contract slightly, and as
they heated up further, would start to convert helium into heavier elements like carbon or oxygen. This, however,
would not release much more energy, so a crisis would occur, as was described in the chapter on black holes. What
happens next is not completely clear, but it seems likely that the central regions of the star would collapse to a very
dense state, such as a neutron star or black hole. The outer regions of the star may sometimes get blown off in a
tremendous explosion called a supernova, which would outshine all the other stars in its galaxy. Some of the heavier
elements produced near the end of the star’s life would be flung back into the gas in the galaxy, and would provide
some of the raw material for the next generation of stars. Our own sun contains about 2 percent of these heavier
elements, because it is a second- or third-generation star, formed some five thousand million years ago out of a
cloud of rotating gas containing the debris of earlier supernovas. Most of the gas in that cloud went to form the sun or
got blown away, but a small amount of the heavier elements collected together to form the bodies that now orbit the
sun as planets like the earth.
The earth was initially very hot and without an atmosphere. In the course of time it cooled and acquired an
atmosphere from the emission of gases from the rocks. This early atmosphere was not one in which we could have
survived. It contained no oxygen, but a lot of other gases that are poisonous to us, such as hydrogen sulfide (the gas
that gives rotten eggs their smell). There are, however, other primitive forms of life that can flourish under such
conditions. It is thought that they developed in the oceans, possibly as a result of chance combinations of atoms into
large structures, called macromolecules, which were capable of assembling other atoms in the ocean into similar
structures. They would thus have reproduced themselves and multiplied. In some cases there would be errors in the
reproduction. Mostly these errors would have been such that the new macromolecule could not reproduce itself and
eventually would have been destroyed. However, a few of the errors would have produced new macromolecules that
were even better at reproducing themselves. They would have therefore had an advantage and would have tended
to replace the original macromolecules. In this way a process of evolution was started that led to the development of
more and more complicated, self-reproducing organisms. The first primitive forms of life consumed various materials,
including hydrogen sulfide, and released oxygen. This gradually changed the atmosphere to the composition that it
has today, and allowed the development of higher forms of life such as fish, reptiles, mammals, and ultimately the
human race.
This picture of a universe that started off very hot and cooled as it expanded is in agreement with all the
observational evidence that we have today. Nevertheless, it leaves a number of important questions unanswered:
1. Why was the early universe so hot?
2. Why is the universe so uniform on a large scale? Why does it look the same at all points of space and in all
directions? In particular, why is the temperature of the microwave back-ground radiation so nearly the same when we
look in different directions? It is a bit like asking a number of students an exam question. If they all give exactly the
same answer, you can be pretty sure they have communicated with each other. Yet, in the model described above,
there would not have been time since the big bang for light to get from one distant region to another, even though the
regions were close together in the early universe. According to the theory of relativity, if light cannot get from one
region to another, no other information can. So there would be no way in which different regions in the early universe
could have come to have the same temperature as each other, unless for some unexplained reason they happened
to start out with the same temperature.
3. Why did the universe start out with so nearly the critical rate of expansion that separates models that recollapse
from those that go on expanding forever, that even now, ten thousand million years later, it is still expanding at nearly
the critical rate? If the rate of expansion one second after the big bang had been smaller by even one part in a
hundred thousand million million, the universe would have recollapsed before it ever reached its present size.
4. Despite the fact that the universe is so uniform and homogeneous on a large scale, it contains local irregularities,
such as stars and galaxies. These are thought to have developed from small differences in the density of the early
universe from one region to another. What was the origin of these density fluctuations?
The general theory of relativity, on its own, cannot explain these features or answer these questions because of its
prediction that the universe started off with infinite density at the big bang singularity. At the singularity, general
relativity and all other physical laws would break down: one couldn’t predict what would come out of the singularity.
As explained before, this means that one might as well cut the big bang, and any events before it, out of the theory,
because they can have no effect on what we observe. Space-time would have a boundary – a beginning at the big
bang.
Science seems to have uncovered a set of laws that, within the limits set by the uncertainty principle, tell us how the
universe will develop with time, if we know its state at any one time. These laws may have originally been decreed by
God, but it appears that he has since left the universe to evolve according to them and does not now intervene in it.
But how did he choose the initial state or configuration of the universe? What were the “boundary conditions” at the
beginning of time?
One possible answer is to say that God chose the initial configuration of the universe for reasons that we cannot
hope to understand. This would certainly have been within the power of an omnipotent being, but if he had started it
off in such an incomprehensible way, why did he choose to let it evolve according to laws that we could understand?
The whole history of science has been the gradual realization that events do not happen in an arbitrary manner, but
that they reflect a certain underlying order, which may or may not be divinely inspired. It would be only natural to
suppose that this order should apply not only to the laws, but also to the conditions at the boundary of space-time
that specify the initial state of the universe. There may be a large number of models of the universe with different
initial conditions that all obey the laws. There ought to be some principle that picks out one initial state, and hence
one model, to represent our universe.
One such possibility is what are called chaotic boundary conditions. These implicitly assume either that the universe
is spatially infinite or that there are infinitely many universes. Under chaotic boundary conditions, the probability of
finding any particular region of space in any given configuration just after the big bang is the same, in some sense,
as the probability of finding it in any other configuration: the initial state of the universe is chosen purely randomly.
This would mean that the early universe would have probably been very chaotic and irregular because there are
many more chaotic and disordered configurations for the universe than there are smooth and ordered ones. (If each
configuration is equally probable, it is likely that the universe started out in a chaotic and disordered state, simply
because there are so many more of them.) It is difficult to see how such chaotic initial conditions could have given
rise to a universe that is so smooth and regular on a large scale as ours is today. One would also have expected the
density fluctuations in such a model to have led to the formation of many more primordial black holes than the upper
limit that has been set by observations of the gamma ray background.
If the universe is indeed spatially infinite, or if there are infinitely many universes, there would probably be some large
regions somewhere that started out in a smooth and uniform manner. It is a bit like the well-known horde of monkeys
hammering away on typewriters – most of what they write will be garbage, but very occasionally by pure chance they
will type out one of Shakespeare’s sonnets. Similarly, in the case of the universe, could it be that we are living in a
region that just happens by chance to be smooth and uniform? At first sight this might seem very improbable,
because such smooth regions would be heavily outnumbered by chaotic and irregular regions. However, suppose
that only in the smooth regions were galaxies and stars formed and were conditions right for the development of
complicated self-replicating organisms like ourselves who were capable of asking the question: why is the universe
so smooth.? This is an example of the application of what is known as the anthropic principle, which can be
paraphrased as “We see the universe the way it is because we exist.”
There are two versions of the anthropic principle, the weak and the strong. The weak anthropic principle states that
in a universe that is large or infinite in space and/or time, the conditions necessary for the development of intelligent
life will be met only in certain regions that are limited in space and time. The intelligent beings in these regions
